Why, according to Jewish law, will relatives be rejected as witnesses in a court of law?
Our Gemara raises this issue while attempting to clarify a case that the Talmud calls kasha she-be’dinei mamonot – one of the most difficult rulings in civil law. The case suggested by the Gemara as being one that is very difficult to understand is a situation where a person signs a legal document as a witness, and at a later time marries and becomes related to one of the people involved in the contract. According to Jewish law, even though he can no longer come to court and attest to the fact that he witnessed and signed the document – since he is a relative and unable to testify – nevertheless, other people who recognize his handwriting can testify that it is his signature and the document will be accepted based on his signature.
The Gemara suggests that this can be explained simply by understanding that a relative is disqualified from testifying not because we are afraid that he will lie, but simply because it is a gezerat melekh – it is the decree of the King (i.e. from the Torah; from God). The proof presented for this idea is that even people with impeccable credentials – like Moshe Rabbeinu and Aharon HaKohen would not be permitted to testify regarding an issue relating to a relative.
A variant reading brought by the Ra’avad and others has the Gemara pointing to the fact that Moshe and Aharon are unable to testify about one-another, indicating that the problem is not dependent on trustworthiness but on the very fact that they are related.
The Ritva points out that it makes no sense to suggest that the underlying basis for relatives to be excluded from testifying is that they have a prejudiced view, given that they love their relative, since Jewish law forbids relatives from testifying not only on behalf of their relatives, but even if the testimony will be against their relatives.